<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div id="app">
    <input name="username" v-model="username">
    {{username}}
</div>

<script src="js/vue.js"></script>
<script>
    new Vue({
        el: "#app",
        data(){
            return{
                username:""
            }
        }
    })
</script>
</body>
</html>